Internal combustion engines are associated with after treatment systems due to stringent emissions regulation. An after treatment system may include one or more after treatment devices provided in an exhaust system of the internal combustion engine. Among these devices, a SCR device or Selective Catalytic Reduction device in which the nitrogen oxides (NOx) contained in the exhaust gas are reduced into diatomic nitrogen (N2) and water (H2O), with the aid of a gaseous reducing agent, typically ammonia (NH3) that can be obtained by urea (CH4N2O) thermo-hydrolysis and that is absorbed inside catalyst.
Typically, urea is contained in a dedicated tank, is injected in the exhaust line and mixed with the exhaust gas upstream of the SCR device. Other fluids can be used in an SCR device in lieu of urea and are generally referred to as Diesel Exhaust Fluids (DEF).
More generally, internal combustion engines are conventionally equipped with a variety of fluid tanks, for example a fuel tank or the above mentioned urea or DEF tank for the SCR exhaust gas treatment system, for which it is desirable to monitor the level of the fluid contained within. Continuous fluid level sensors have widely been used for detecting the level of fluid in a tank. These sensors work by continuously measuring a level within a specified range and determining the exact amount of fluid in the tank as a function of the measured level. Unfortunately these sensors are relatively expensive.
As an alternative to continuous sensors, discrete level sensors may also be used. Discrete level sensors provide information on the level of fluid in the tank by simply indicating whether the fluid in the tank is above or below predetermined level threshold values. Discrete level sensors are therefore less precise since they are unable to detect the precise level of fluid between two level threshold values.
Moreover, when used in internal combustion engines they present additional problems. During transitory driving states of the vehicle, i.e. for example during acceleration or deceleration, discrete level sensors provide level indication which are often misleading. In those situations the fluid in the tank is sloshed around and the level threshold values are randomly exceeded so that the sensor provides conflicting information regarding the actual level of the fluid in the tank. This is even more true when the discrete level sensors are used for example to detect the level of urea in urea tanks. Such tanks normally have a relatively flat and wide parallelepiped shape and small movements of the vehicle are enough to cause the fluid to slosh in the tank and to randomly exceed or fall below various threshold level values.
It is also important to note that, in connection with the use of an SCR device, in case of low level of DEF in the tank, warnings are displayed to the driver, for example the information of the remaining distance that the vehicle can travel. Also, on the basis of the remaining DEF quantity, some inducement actions may be activated. Inducement action may include vehicle speed performance limitation that is annoying to the driver and, if the DEF quantity is lower than a predefined threshold, engine restart prevention. These inducement actions are designed to avoid the use of the vehicle and induce the driver to refill the DEF tank. Furthermore, current legislation requires that all warnings and inducement actions can be switched off only in case the customer does a tank refill. Therefore an SCR system equipped with a DEF tank and a level sensor must be able to detect if a refill has been done, in order to disable all inducement strategies for the driver.